QuickTime 3 Reference
Prior to QuickTime 2.1, when displaying the output of a sequence grabber video channel in a window, an application would have to pause the sequence grabber before moving or resizing a window, before a menu was pulled down, or whenever the application was put into the background. If an application failed to take these precautions, it was possible for the digitized video to draw outside of the window with which it was associated.
QuickTime 2.1 solves these problems so that an application using a sequence grabber video channel in a window no longer has to take any special precautions to ensure that the video remains within the window. As long as the application calls SGIdle regularly, the sequence grabber will automatically take care of pausing and unpausing the video as necessary.